Zoned diesel oxidation catalyst

ABSTRACT

An oxidation catalyst is described for treating an exhaust gas from a diesel engine, which oxidation catalyst comprises: a substrate; a first washcoat region disposed on the substrate, wherein the first washcoat region comprises a first platinum group metal (PGM) and a first support material; a second washcoat region adjacent to the first washcoat region, wherein the second washcoat region comprises a second platinum group metal (PGM) and a second support material; a third washcoat region disposed on the substrate, wherein the third washcoat region comprises a third platinum group metal (PGM) and a third support material; and wherein either: (i) the third washcoat region is adjacent to the second washcoat region; or (ii) the second washcoat region is disposed or supported on the third washcoat region. Also described are uses and methods involving the oxidation catalyst.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 14/445,125,which claims priority benefit to U.S. Provisional Patent Application No.61/860,608 filed on Jul. 31, 2013, to Great Britain Patent ApplicationNo. 1315892.8 filed on Sep. 6, 2013 and to Great Britain PatentApplication No. 1316278.9 filed on Sep. 12, 2013, all of which areincorporation herein by reference.

FIELD OF THE INVENTION

The invention relates to an oxidation catalyst for a diesel engine, itsuses and to a method of its production. The invention also relates tomethods involving the oxidation catalyst. The invention further providesan exhaust system comprising the oxidation catalyst, and to a vehiclecomprising the oxidation catalyst or the exhaust system.

BACKGROUND TO THE INVENTION

Diesel engines (also referred to as compression ignition engines)produce an exhaust emission that generally contains at least fourclasses of pollutant that are legislated against by inter-governmentalorganisations throughout the world: carbon monoxide (CO), unburnedhydrocarbons (HCs), oxides of nitrogen (NO_(x)) and particulate matter(PM).

Oxidation catalysts comprising platinum group metals (PGMs) have beenused to treat carbon monoxide (CO) and hydrocarbons (HCs), including thevolatile organic fraction (VOF) of particulate matter (PM), in exhaustemissions produced by diesel engines. Such catalysts treat carbonmonoxide (CO) by oxidising it to carbon dioxide (CO₂), and treathydrocarbons (HCs) by oxidising them to water (H₂O) and carbon dioxide(CO₂). Some platinum group metals, particularly when supported on arefractory oxide, can also promote the oxidation of nitric oxide (NO) tonitrogen dioxide (NO₂).

As emissions standards for permissible emission of pollutants fromdiesel engines, particularly vehicular engines, become progressivelytightened, there is a need to provide improved exhaust systems that areable to meet these standards and which are cost-effective. It isdesirable to provide an oxidation catalyst that has excellent activitytoward CO and HCs, and which is not readily poisoned by sulphur indiesel fuel. It is important that the oxidation catalyst works inconjunction with other emissions control devices, particularly as partof an exhaust system to maximise the overall reduction in pollutantsproduced by a diesel engine.

Platinum group metals (PGMs) are expensive metals. Catalystmanufacturers are under pressure to maximise the effectiveness of anyPGMs that are included in an oxidation catalyst to minimise its materialcost. The method of producing the oxidation catalyst is also importantbecause there is a manufacturing cost associated with each step ofapplying a washcoat to a substrate and to the subsequent drying andcalcination steps.

Our earlier WO 2007/077462 describes an exhaust system for a lean-burninternal combustion engine comprising a catalyst for oxidising carbonmonoxide (CO) and hydrocarbons (HCs), where the catalyst comprises afirst washcoat zone containing at least one PGM, which first washcoatzone being defined at an upstream end by an inlet end of a substratemonolith and at a downstream end by a point less than half way along alength of the substrate monolith measured from the inlet end; a secondwashcoat zone containing at least one PGM, which second washcoat zonecomprising the point half way along the substrate monolith lengthmeasured from the inlet end; and a third washcoat zone containing atleast one PGM, which third washcoat zone being defined at a downstreamend by an outlet end of the substrate monolith and at an upstream end bya point at most three quarters of the way along the substrate monolithlength from the inlet end, wherein both the PGM loading in the firstwashcoat zone and the PGM loading in the third washcoat zone is greaterthan the PGM loading in the second washcoat zone and wherein the firstwashcoat zone comprises a washcoat loading that is less than a washcoatloading of the third washcoat zone.

SUMMARY OF THE INVENTION

The inventors have devised an oxidation catalyst having an arrangementthat can be manufactured in a cost-effective manner and which maximisesthe effectiveness of its component PGMs. The oxidation catalyst providesexcellent CO and HC oxidation activity, and is tolerant to sulphur indiesel fuel and its oxides. The oxidation catalyst also shows goodactivity toward oxidising nitric oxide (NO) to nitrogen dioxide (NO₂).

The invention provides an oxidation catalyst for treating an exhaust gasfrom a diesel engine, which oxidation catalyst comprises:

-   -   a substrate;    -   a first washcoat region disposed or supported on the substrate,        wherein the first washcoat region comprises a first platinum        group metal (PGM) and a first support material;    -   a second washcoat region adjacent to the first washcoat region,        wherein the second washcoat region comprises a second platinum        group metal (PGM) and a second support material;    -   a third washcoat region disposed or supported on the substrate,        wherein the third washcoat region comprises a third platinum        group metal (PGM) and a third support material; and wherein        either:

(i) the third washcoat region is adjacent to the second washcoat region;or

(ii) the second washcoat region is disposed or supported on the thirdwashcoat region.

When the second and third washcoat regions are arranged as described in(i) or (ii), exhaust gas entering the oxidation catalyst will primarilycome into contact with the second washcoat region before the thirdwashcoat region. This arrangement in the oxidation catalyst of theinvention can be used to provide good tolerance to poisoning by sulphurin diesel fuel and to optimise the oxidation of nitric oxide (NO) tonitrogen dioxide (NO₂). In particular, the oxidation activity of thecatalyst toward HCs can be maintained, even after some sulphation hasoccurred.

The invention also provides an exhaust system for a diesel engine. Theexhaust system comprises an oxidation catalyst of the invention and anemissions control device.

The oxidation of nitric oxide (NO) to nitrogen dioxide (NO₂) by theoxidation catalyst can be important for the overall removal ofpollutants by an exhaust system. The ratio of NO₂ to NO in an exhaustgas can affect the active or passive regeneration of an emissionscontrol device that comprises a filtering substrate (e.g. dieselparticulate filter (DPF), catalysed soot filter (CSF), selectivecatalytic reduction filter (SCRF™)). NO₂ in an exhaust gas can assist inthe oxidation of particulate matter (PM) collected by a filteringsubstrate of a downstream emissions control device. The oxidationcatalyst of the invention is particularly suitable for use in the activeregeneration of an emissions control device comprising a filteringsubstrate.

The amount of NO₂ in an exhaust gas can also affect the performance ofan emissions control device that is downstream of the oxidationcatalyst. Selective catalytic reduction (SCR) catalysts and selectivecatalytic reduction filter (SCRF™) catalysts for treating NO_(x) (e.g.NO₂+NO) often require the ratio of NO₂ to NO in the inlet gas to bewithin a specific range for optimum catalytic performance. The optimalNO₂ proportion of NO_(x) typically depends on the type of compositionused in the SCR or SCRF™ catalyst. The oxidation catalyst of theinvention can be used with an SCR or SCRF™ catalyst, particularly an SCRor SCRF™ catalyst comprising a copper exchanged zeolite.

The invention further relates to a vehicle. The vehicle comprises adiesel engine and either an oxidation catalyst of the invention or anexhaust system of the invention.

The invention further provides methods of producing an oxidationcatalyst of the invention.

In a first method aspect of producing the oxidation catalyst, the methodcomprises:

-   (i) coating a substrate with a first washcoat of length L₁, wherein    the substrate has an axial length L and L₁ is less than or equal to    the axial length L (e.g. L₁≤L); then-   (ii) coating the substrate with a second washcoat of length L₂,    wherein L₂ is less than or equal to the axial length L (e.g. L₂≤L);-   (iii) drying the first washcoat and the second washcoat onto the    substrate;-   (iv) impregnating at least one of the first washcoat and the second    washcoat with a platinum group metal to a length L₃, wherein L₃ is    less than the axial length L (e.g. L₃<L); and-   (v) calcining the substrate coated with the first washcoat, the    second washcoat and the impregnated platinum group metal.

In a second method aspect of producing the oxidation catalyst, themethod comprises:

-   (i) coating a substrate from a first end with a first washcoat of    length L₁, wherein the substrate has an axial length L and L₁ is    less than the axial length L (e.g. L₁<L); then-   (ii) coating the substrate from a second end with a second washcoat    of length L₂, wherein L₂ is greater than the difference between L    and L₁ (e.g. L₂>L−L₁); and-   (iii) calcining the substrate coated with the first washcoat and the    second washcoat.

Another aspect of the invention relates to a method of treating anexhaust gas from a diesel engine. The method comprises contacting theexhaust gas with an oxidation catalyst of the invention. The method oftreating an exhaust gas from a diesel engine is a method of treating(e.g. oxidising) carbon monoxide (CO) and hydrocarbons (HCs) in anexhaust gas from a diesel engine, such as by oxidising carbon monoxide(CO), hydrocarbons (HCs) and nitric oxide (NO). Typically, the treatedexhaust gas is then passed onto an emissions control device (i.e.downstream emissions control device). This aspect of the invention alsorelates to the use of an oxidation catalyst of the invention to treat anexhaust gas from a diesel engine, optionally in combination with anemissions control device.

The invention also provides a method of modulating the content ofnitrogen oxides (NO_(x)) in an exhaust gas from a diesel engine for anemissions control device, which method comprises: (a) controlling theNO_(x) content of an exhaust gas by contacting the exhaust gas with anoxidation catalyst of the invention to produce a treated exhaust gas;and (b) passing the treated exhaust gas to an emissions control device.This aspect of the invention also relates to the use of an oxidationcatalyst of the invention to modulate the content of nitrogen oxides(NO_(x)) in an exhaust gas from a diesel engine for an emissions controldevice (i.e. downstream emissions control device).

Another aspect of the invention relates to the use of an oxidationcatalyst of the invention in the regeneration of an emissions controldevice having a filtering substrate. The oxidation catalyst may be usedin the active or passive regeneration of the emissions control device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 4 are representations of oxidation catalysts of theinvention.

FIG. 1 shows an oxidation catalyst comprising a first washcoat region(1), a second washcoat region (2) adjacent to the first washcoat region(1), and a third washcoat region (3) adjacent to the second washcoatregion (2), which are disposed on a substrate (5).

FIG. 1 also provides a schematic representation of the oxidationcatalysts shown in FIGS. 3 and 4.

FIG. 2 shows an oxidation catalyst comprising a first washcoat region(1), a second washcoat region (2) and a third washcoat region (3). Thefirst washcoat region is disposed directly onto a substrate (5). Thesecond washcoat region is disposed on the third washcoat region. Boththe second washcoat region and third washcoat region are adjacent to thefirst washcoat region.

FIGS. 3 and 4 show oxidation catalysts where a second washcoat region(2) is the region of overlap between two washcoat layers (6 and 7) thatare disposed on a substrate (5). In FIG. 3, an end of the washcoat layer(6) that forms a first washcoat region (1) overlaps an end of a washcoatlayer (7) that forms a third washcoat region (3).

In FIG. 4, an end of an end of the washcoat layer (6) that forms a firstwashcoat region (1) is overlapped by an end of a washcoat layer (7) thatforms a third washcoat region (3).

FIG. 5 is a graph showing % NO₂ content of NO_(x) from oxidation of NOby the Reference DOC (♦) and Zoned DOC (▴) at various temperatures.

FIGS. 6 and 7 show the results of quench tests performed on theReference DOC and Zoned DOC respectively.

FIGS. 8 and 9 are histograms showing the amount of HC slip for theReference DOC and Zoned DOC.

FIG. 10 is a histogram showing % NO₂ content of NO_(x) from oxidation ofNO after the Reference DOC and Zoned DOC have each been exposed tosulphur in diesel fuel.

DETAILED DESCRIPTION OF THE INVENTION

Typically, the oxidation catalyst comprises a total amount by mass ofthe platinum group metal (PGM) component of 2.0 to 8.0 g. The totalamount of PGM component that is used depends on, amongst other things,the size of the substrate and the intended application of the oxidationcatalyst.

Generally, the oxidation catalyst comprises a ratio of the total mass ofplatinum to the total mass of the palladium of 3:1 to 1:3 (e.g. 2:1 to1:2), such as 1.5:1 to 1:1.5. It is preferred that the total mass ofplatinum is greater than the total mass of palladium. Preferably, theratio of the total mass of platinum to the total mass of palladium is3:1 to 1.1:1 (e.g. 2.5:1 to 7:6), such as 2:1 to 1.25:1.

The oxidation catalyst generally has a total loading of platinum groupmetal of from 20 to 200 g ft⁻³, preferably 25 to 175 g ft⁻³, and morepreferably 30 to 160 g ft⁻³.

As shown schematically in the Figures, the oxidation catalyst of theinvention comprises a substrate (5); a first washcoat region (1)disposed on the substrate (5); a second washcoat region (2) adjacent tothe first washcoat region (1); and a third washcoat region (3) disposedon the substrate (5). The third washcoat region (3) may be adjacent tothe second washcoat region (3) (see FIG. 1) or the second washcoatregion (2) may be disposed on the third washcoat region (3) (see FIG.2). The second washcoat region (2) is adjacent to the first washcoatregion (1) on the substrate (e.g. the second washcoat region is disposednext to the first washcoat region along the length of the substrate).

For the avoidance of doubt, the first washcoat region (1), the secondwashcoat region (2) and the third washcoat region (3) are supported onthe same substrate.

The oxidation catalyst may comprise an underlying washcoat layerdisposed (e.g. directly disposed or supported) on the substrate. Thefirst washcoat region and the third washcoat region may be directlydisposed or supported on the underlying washcoat layer. When theoxidation catalyst has an arrangement where (i) the third washcoatregion is adjacent to the second washcoat region, then the secondwashcoat region may be directly disposed or supported on the underlyingwashcoat layer.

The underlying washcoat layer typically has a length of at least 90% ofthe length of the substrate, such as 95 to 100% of the length of thesubstrate. It is preferred that the underlying washcoat layer has alength that is the same as the length of the substrate.

However, the first washcoat region is, in general, directly disposed(i.e. directly supported) on the substrate. This means that the firstwashcoat region is in contact with a surface of the substrate. It ispreferred that there is no intermediary washcoat region (e.g. theunderlying washcoat layer) between a surface of the substrate and thefirst washcoat region. More preferably, there is no intermediarywashcoat region (e.g. the underlying washcoat layer) between a surfaceof the substrate and the first washcoat region and the third washcoatregion (and also the second washcoat region when the oxidation catalysthas an arrangement where (i) the third washcoat region is adjacent tothe second washcoat region).

The first washcoat region is typically upstream of the second washcoatregion (i.e. the first washcoat region is nearer to an inlet end of thesubstrate than the second washcoat region is to the inlet end). Theexhaust gas entering the oxidation catalyst will come into contact withthe first washcoat region before it comes into contact with the secondwashcoat region. It is preferred that the first washcoat region isupstream of both the second washcoat region and the third washcoatregion.

The first washcoat region of the oxidation catalyst of the invention isfor initiating oxidation of hydrocarbons (HCs) and/or carbon monoxide(CO) in the exhaust gas, particularly hydrocarbons (HCs). Thus, thefirst washcoat region has a low HC light off temperature and/or CO lightoff temperature (e.g. a low HC T₅₀ and/or a low CO T₅₀). The activity ofthe first washcoat region is advantageous for relatively low exhaust gastemperatures, such as shortly after starting a diesel engine. Theoxidation of HCs and/or CO by the first washcoat region generates anexotherm, which can increase the temperature of the exhaust gas and thebody of the catalyst to a temperature that is optimal for catalysis bythe second and third washcoat regions.

Generally, the first washcoat region comprises, or consists of, a firstwashcoat layer. The first washcoat layer may be directly disposed (i.e.directly supported) on the substrate.

The first washcoat region may comprise, or consist of, a first washcoatzone. The first washcoat zone may comprise, or consist of, a firstwashcoat layer. Thus, the first washcoat zone comprises, or consists of,the whole or complete length of the first washcoat layer.

The first washcoat zone may comprise, or consist of, a first washcoatlayer and a fourth washcoat layer, wherein the fourth washcoat layer isdisposed on the first washcoat layer. Thus, the first washcoat zonecomprises, or consists of, the whole or complete length of the firstwashcoat layer and the whole or complete length of the fourth washcoatlayer.

The first washcoat region may comprise, or consist of, a part or portionof the first washcoat layer (see (1) and (6) in FIGS. 3 and 4). The partof the first washcoat layer is a part length (e.g. not the whole orcomplete length) of the first washcoat layer.

When the first washcoat region comprises, or consists of, a part of thefirst washcoat layer, then typically there is an overlap between thefirst washcoat layer and a third washcoat layer (e.g. the third washcoatlayer as defined below).

Additionally or alternatively, when the first washcoat region comprises,or consists of, a part of the first washcoat layer, then the firstwashcoat region or the part of the first washcoat layer may comprise aPGM zone. The part of the first washcoat layer (i.e. that forms thefirst washcoat region) may comprise, or consist of, a PGM zone and anadjacent part or portion of the first washcoat layer (i.e. the part orportion of the first washcoat layer is adjacent to the PGM zone on thesubstrate). The PGM zone can be prepared in accordance with the firstmethod aspect of the invention. The PGM zone may be formulated toprovide high oxidation activity toward hydrocarbons, such as higheractivity than the remainder of the oxidation catalyst.

The length of the PGM zone is typically less than the length of thefirst washcoat region or the part of the first washcoat layer (i.e. thatforms the first washcoat region). Thus, the first washcoat region orpart of the first washcoat layer may be subdivided into a PGM zone andan adjacent part or portion of the first washcoat layer.

In general, the PGM zone is at or an adjacent to an inlet end of thesubstrate. The PGM is preferably upstream of the adjacent part orportion of the first washcoat layer.

The first washcoat region may comprise a first washcoat layer and afourth washcoat layer, wherein the fourth washcoat layer is disposed onthe first washcoat layer. When the first washcoat region comprises afirst washcoat layer and a fourth washcoat layer, then preferably thefirst washcoat region comprises a part or portion of the first washcoatlayer and a part or portion of the fourth washcoat layer, wherein thepart of the fourth washcoat layer is disposed on the part of the firstwashcoat layer. The part of the first washcoat layer is a part length(e.g. not the whole or complete length) of the first washcoat layer. Thepart of the fourth washcoat layer is a part length (e.g. not the wholeor complete length) of the fourth washcoat layer.

Generally, the first washcoat region or first washcoat zone has a lengththat is less than the length (i.e. axial length L) of the substrate. Itis preferred that the first washcoat region or first washcoat zone has alength of at least 5% of the length of the substrate. The first washcoatregion or first washcoat zone typically has a length of 10 to 80% of thelength of the substrate, preferably 20 to 70%, more preferably 30 to 60%of the length of the substrate, such as 40 to 60% of the length of thesubstrate.

It is generally preferred that the first washcoat region or firstwashcoat zone is less than or equal to 50% of the length of thesubstrate, preferably less than 50% of the length of the substrate, morepreferably less than 30% of the length of the substrate. Thus, the firstwashcoat region or first washcoat zone preferably has a length of 10 to50% of the length of the substrate, more preferably 15 to 45% of thelength of the substrate, such as 20 to 40% of the length of thesubstrate, and even more preferably 25 to 35% of the length of thesubstrate.

When the first washcoat region or the part of the first washcoat layercomprises a PGM zone, then the length of the PGM zone is typically 1 to25% of the length of the substrate, preferably 2.5 to 20% of the lengthof the substrate (e.g. 2.5 to 7.5% of the length of the substrate, morepreferably 5 to 15% of the length of the substrate (e.g. 5 to 10% of thelength of the substrate).

The PGM zone may have a length of 0.5 to 2 inches (12.7 to 50.81 mm). Itis preferred that the PGM zone has a length of 0.5 to 1.75 inches, suchas 0.5 to 1.5 inches or 0.6 to 1.75 inches, more preferably 0.75 to 1.25inches. Typically, the PGM zone has a length of about or approximately 1inch (2.54 cm).

Generally, the first washcoat region or the first washcoat zone isdisposed at or adjacent to an inlet end of the substrate.

The first washcoat region, the first washcoat zone or the first washcoatlayer comprises a first platinum group metal (PGM). It is preferred thatthe only PGM in the first washcoat region is the first PGM.

The first PGM is typically selected from the group consisting ofplatinum, palladium and a combination of platinum and palladium. Thefirst PGM may be platinum (e.g. platinum only). The first PGM may bepalladium (e.g. palladium only). The first PGM may be a combination ofplatinum and palladium (e.g. platinum and palladium only).

In general, the total loading of the first PGM (e.g. of the firstwashcoat region, zone or layer) is greater than the total loading of thethird PGM (e.g. of the third washcoat region, zone or layer).

The first washcoat region typically has a total loading of the first PGMof 5 to 300 g ft⁻³. Preferably, the total loading of the first PGM is 10to 250 g ft⁻³, more preferably 15 to 200 g ft⁻³ (e.g. 15 to 100 g ft⁻³),still more preferably 20 to 150 g ft⁻³ (e.g. 20 to 75 g ft⁻³) and evenmore preferably 25 to 100 g ft⁻³. When the first washcoat regioncomprises, or consists of, a first washcoat layer or a first washcoatzone, then preferably the first washcoat layer or first washcoat zonehas a total loading of the first PGM of 5 to 300 g ft⁻³. Preferably, thetotal loading of the first PGM is 10 to 250 g ft⁻³, more preferably 15to 200 g ft⁻³ (e.g. 15 to 100 g ft⁻³), still more preferably 20 to 150 gft⁻³ (e.g. 20 to 75 g ft⁻³) and even more preferably 25 to 100 g ft⁻³.

When the first PGM is a combination of platinum and palladium, thentypically the first washcoat region, the first washcoat zone or thefirst washcoat layer comprises a ratio of the total mass of platinum tothe total mass of palladium of 3.5:1 to 1:3.5 (e.g. 3:1 to 1:3),preferably 2:1 to 1:2, and more preferably 1.5:1 to 1:1.5 (e.g. about1:1).

It is preferred that when the first PGM is a combination of platinum andpalladium then the first washcoat region, the first washcoat zone or thefirst washcoat layer comprises a total mass of platinum that is greaterthan or equal to the total mass of palladium (e.g. the ratio of thetotal mass of platinum to the total mass of palladium is ≥1:1).Advantageous light off activity can be obtained when the total mass ofplatinum is greater than or equal to the total mass of palladium in thefirst washcoat region. More preferably, the ratio of the total mass ofplatinum to the total mass of palladium is 3.5:1 to 1:1 (e.g. 3.5:1 to1.1:1), preferably 3:1 to 1.25:1, and more preferably 2.5:1 to 1.5:1.

When the first PGM is palladium or a combination of platinum andpalladium, then the first washcoat region, the first washcoat zone orthe first washcoat layer may comprise gold. The first washcoat region,the first washcoat zone or the first washcoat layer may comprise apalladium-gold alloy (e.g. the palladium of the first platinum groupmetal may be present as an alloy with gold). Catalysts comprising gold(Au) can be prepared using the method described in WO 2012/120292.

When the first washcoat region, the first washcoat zone or the firstwashcoat layer comprises gold, such as a palladium-gold alloy, thengenerally the first washcoat region, the first washcoat zone or thefirst washcoat layer comprises a ratio of the total mass of palladium(Pd) to the total mass of gold (Au) of 9:1 to 1:9, preferably 5:1 to1:5, and more preferably 2:1 to 1:2.

The PGM zone typically comprises a platinum group metal (PGM) selectedfrom the group consisting of platinum, palladium and a combinationthereof. The definition of the first PGM comprises the platinum groupmetal (PGM) of the PGM zone.

The PGM zone comprises a different total amount (e.g. total loading) ofPGM than the remainder of the first washcoat region (i.e. the adjacentpart of the first washcoat layer). It is preferred that the PGM zone hasa total amount (e.g. total loading) of PGM that is greater than theremainder of the first washcoat region.

It is preferred that the total loading of platinum (Pt) in the PGM zoneis greater than the total loading of platinum (Pt) in the remainder ofthe first washcoat region (i.e. the adjacent part of the first washcoatlayer). More preferably, the total loading of palladium (Pd) in the PGMzone is less than the total loading of palladium (Pd) in the remainderof the first washcoat region (i.e. the adjacent part of the firstwashcoat layer).

When the PGM zone comprises platinum (Pt) and palladium (Pd), thentypically the weight ratio of platinum (Pt) to palladium (Pd) is ≥1:1.It is preferred that the weight ratio of platinum (Pt) to palladium (Pd)is ≥1.1:1, more preferably ≥1.25:1, particularly ≥1.5:1, such as ≥1.75:1(e.g. ≥2:1), and still more preferably ≥2.5:1 (e.g. ≥5:1). Thus, the PGMzone typically comprises platinum (Pt) and palladium (Pd) in a weightratio of 10:1 to 1:1 (e.g. 2:1 to 1.1:1 or 7.5:1 to 5:1), morepreferably 8:1 to 1.25:1 (e.g. 7:1 to 1.5:1), and still more preferably6:1 to 2.5:1.

Typically, the first support material comprises, or consists essentiallyof, a refractory metal oxide. Refractory metal oxides suitable for useas a support material in an oxidation catalyst for a diesel engine arewell known in the art. The refractory metal oxide may be selected fromthe group consisting of alumina, silica, titania, zirconia, ceria andmixed or composite oxides of two or more thereof. It is preferred thatthe refractory metal oxide is selected from alumina, silica and mixed orcomposite oxides thereof. More preferably, the refractory metal oxide isselected from alumina and silica-alumina. Even more preferably, therefractory metal oxide is alumina.

When the first PGM is palladium or a combination of platinum andpalladium, then the first support material may or may not comprise, orconsist of, cerium oxide, particularly ceria (CeO₂) or ceria-zirconia(CeO₂—ZrO₂).

In general, the first PGM is disposed or supported on the first supportmaterial. When the first washcoat region, the first washcoat zone or thefirst washcoat layer comprises gold, then the gold or the palladium-goldalloy may be disposed or supported on the first support material. Forexample, the first PGM and/or gold can be dispersed on the first supportmaterial and/or impregnated into the first support material.

The first washcoat region, the first washcoat layer or the firstwashcoat zone typically comprises a total washcoat loading of 0.25 to 5g in⁻³, preferably 0.5 to 4 g in⁻³, such as 0.75 to 3 g in⁻³, morepreferably 1.0 to 2.5 g in⁻³, such as 1.25 to 2.0 g in⁻³.

The first washcoat region, the first washcoat zone or the first washcoatlayer may further comprise a hydrocarbon adsorbent. The hydrocarbonadsorbent may be selected from a zeolite, active charcoal, porousgraphite and a combination of two or more thereof. It is preferred thatthe hydrocarbon adsorbent is a zeolite.

When the first washcoat region, the first washcoat zone or the firstwashcoat layer comprises a hydrocarbon adsorbent, then typically thetotal amount of hydrocarbon adsorbent is 0.05 to 3.00 g in⁻³,particularly 0.10 to 2.00 g in⁻³ (e.g. 0.2 to 0.8 g in⁻³).

The first washcoat region, the first washcoat zone or the first washcoatlayer may or may not comprise an alkaline earth metal.

Typically, the alkaline earth metal comprises magnesium (Mg), calcium(Ca), strontium (Sr), barium (Ba) or a combination of two or morethereof. It is preferred that the alkaline earth metal comprises calcium(Ca), strontium (Sr), or barium (Ba), more preferably strontium (Sr) orbarium (Ba), and most preferably the alkaline earth metal comprisesbarium (Ba).

Typically, the amount of the alkaline earth metal is 0.07 to 3.75 molft⁻³, particularly 0.1 to 3.0 mol ft⁻³, more particularly 0.2 to 2.5 molft⁻³ (e.g. 0.25 to 1.0 mol ft⁻³), such as 0.3 to 2.25 mol ft⁻³,especially 0. 0.35 to 1.85 mol ft⁻³, preferably 0.4 to 1.5 mol ft⁻³,more preferably 0.5 to 1.25 mol ft⁻³.

When the first washcoat region, the first washcoat zone or the firstwashcoat layer further comprises an alkaline earth metal, then the firstsupport material may or may not comprise, or consist essentially of,alumina doped with a heteroatom component. The heteroatom componenttypically comprises silicon, magnesium, barium, lanthanum, cerium,titanium, or zirconium or a combination of two or more thereof. Theheteroatom component may comprises, or consist essentially of, an oxideof silicon, an oxide of magnesium, an oxide of barium, an oxide oflanthanum, an oxide of cerium, an oxide of titanium or an oxide ofzirconium. More preferably, the alumina doped with a heteroatomcomponent is alumina doped with silica or alumina doped with magnesiumoxide. Even more preferably, the alumina doped with a heteroatomcomponent is alumina doped with silica. Alumina doped with a heteroatomcomponent can be prepared using methods known in the art or, forexample, by a method described in U.S. Pat. No. 5,045,519.

The first washcoat region, the first washcoat zone or the first washcoatlayer may consist essentially of a first PGM, a first support materialand an alkaline earth metal component.

The first washcoat region, the first washcoat zone or the first washcoatlayer preferably consists essentially of a first PGM and a first supportmaterial.

The second washcoat region is adjacent to the first washcoat region. Thesecond washcoat region is either (i) adjacent (e.g. directly adjacent)to the third washcoat region or (ii) the second washcoat region isdisposed on the third washcoat region.

Exhaust gas entering the catalyst of the invention generally comes intocontact with the second washcoat region before the third washcoatregion. The second washcoat region acts as a “stabilizer” for the firstwashcoat region. The second washcoat region acts as a “stabilizer” inthe sense that it performs some or all of the oxidation reactions of thefirst washcoat region, but the second washcoat region may have a higherlight off temperature for HC and/or CO than that of the first washcoatregion. This is to ensure that the oxidation of HCs and/or CO iscontinued at higher exhaust gas temperatures and it may generate anadditional exotherm.

The second washcoat region may also provide two other functions: (1) tooxidise NO to NO₂, particularly when the second PGM is platinum only,and/or (2) to protect the third washcoat region from being poisoned bysulphur in diesel fuel or the oxides thereof, particularly when thethird PGM of the third washcoat region comprises palladium. Platinum isgenerally more resistant than palladium to poisoning by sulphur.Platinum also shows excellent NO oxidation activity. When, for example,the second PGM is platinum (i.e. only platinum), then it may protect thethird washcoat region from sulphur poisoning that occurs at highertemperatures, thereby improving the sulphur tolerance of the oxidationcatalyst as a whole.

Generally, the second washcoat region has a different composition (i.e.a different overall composition) to the first washcoat region.

Typically, the second washcoat region has a length that is less than thelength (i.e. axial length L) of the substrate. It is preferred that thesecond washcoat region has a length of at least 10% of the length of thesubstrate, more preferably at least 15% of the length of the substrate,such as at least 20% of the length of the substrate.

The second washcoat region may have a length of 10 to 70% of the lengthof the substrate (e.g. 40 to 70%), such as 15 to 60% of the length ofthe substrate, particularly, 20 to 50% of the length of the substrate,preferably 30 to 40% of the length of the substrate.

The second washcoat region may comprise, or consist of, a secondwashcoat zone.

The second washcoat region may comprise, or consist of, a secondwashcoat layer.

The second washcoat region, the second washcoat zone or the secondwashcoat layer typically abuts or is contiguous with the first washcoatregion.

The second washcoat region comprises a second platinum group metal(PGM). It is preferred that the only PGM in the second washcoat regionis the second PGM.

Typically, the second PGM is selected from the group consisting ofplatinum, palladium and a combination of platinum and palladium. Thesecond PGM may be platinum (e.g. platinum only). The second PGM may bepalladium (e.g. palladium only). The second PGM may be a combination ofplatinum and palladium (e.g. platinum and palladium only).

The second washcoat region typically has a total loading of the secondPGM of 5 to 300 g ft⁻³. Preferably, the total loading of the second PGMis 10 to 250 g ft⁻³, more preferably 15 to 200 g ft⁻³ (e.g. 15 to 100 gft⁻³), still more preferably 20 to 150 g ft⁻³ (e.g. 20 to 75 g ft⁻³) andeven more preferably 25 to 100 g ft⁻³. When the second washcoat regioncomprises, or consists of, a second washcoat layer or a second washcoatzone, then preferably the second washcoat layer or the second washcoatzone has a total loading of the second PGM of 10 to 250 g ft⁻³, morepreferably 15 to 200 g ft⁻³ (e.g. 15 to 100 g ft⁻³), still morepreferably 20 to 150 g ft⁻³ (e.g. 20 to 75 g ft⁻³), such as 25 to 100 gft⁻³.

Generally, when the second PGM is a combination of platinum andpalladium, the second washcoat region, the second washcoat zone or thesecond washcoat layer comprises a ratio of the total mass of platinum tothe total mass of palladium of 10:1 to 1:10 (e.g. 7.5:1 to 1:7.5),preferably 5:1 to 1:5 (e.g. 3:1 to 1:3), and more preferably 2.5:1 to1:2.5 (e.g. 2:1 to 1:2).

When the second PGM is palladium or a combination of platinum andpalladium, then the second washcoat region, the second washcoat zone orthe second washcoat layer may comprise gold. The second washcoat region,the second washcoat zone or the second washcoat zone may comprise apalladium-gold alloy (e.g. the palladium of the second PGM may be analloy with gold).

When the second washcoat region, the second washcoat zone or the secondwashcoat layer comprises gold, such as a palladium-gold alloy, thengenerally the second washcoat region, the second washcoat zone or thesecond washcoat layer comprises a ratio of the total mass of palladium(Pd) to the total mass of gold (Au) of 9:1 to 1:9, preferably 5:1 to1:5, and more preferably 2:1 to 1:2.

Typically, the second support material comprises, or consistsessentially of, a refractory metal oxide. The refractory metal oxide maybe selected from the group consisting of alumina, silica, titania,zirconia, ceria and mixed or composite oxides of two or more thereof. Itis preferred that the refractory metal oxide is selected from alumina,silica and mixed or composite oxides thereof. More preferably, therefractory metal oxide is selected from alumina and silica-alumina. Evenmore preferably, the refractory metal oxide is alumina.

In general, the second PGM is disposed or supported on the secondsupport material. When the second washcoat region, the second washcoatzone or the second washcoat layer comprises gold, then the gold or thepalladium-gold alloy may be disposed or supported on the second supportmaterial. For example, the second PGM and/or gold can be dispersed onthe second support material and/or impregnated into the second supportmaterial.

When the second PGM is palladium or a combination of platinum andpalladium, then the second support material may or may not comprise, orconsist of, cerium oxide, particularly ceria (CeO₂) or ceria-zirconia(CeO₂—ZrO₂).

The second washcoat region, the second washcoat layer or the secondwashcoat zone typically comprises a total washcoat loading of 0.25 to 5g in⁻³, preferably 0.5 to 4 g in⁻³, such as 0.75 to 3 g in⁻³, morepreferably 1.0 to 2.5 g in⁻³, such as 1.25 to 2.0 g in⁻³.

The second washcoat region, the second washcoat zone or the secondwashcoat layer may further comprise a hydrocarbon adsorbent. Thehydrocarbon adsorbent may be selected from a zeolite, active charcoal,porous graphite and a combination of two or more thereof. It ispreferred that the hydrocarbon adsorbent is a zeolite.

When the second washcoat region, the second washcoat zone or the secondwashcoat layer comprises a hydrocarbon adsorbent, then typically thetotal amount of hydrocarbon adsorbent is 0.05 to 3.00 g in⁻³,particularly 0.10 to 2.00 g in⁻³, more particularly 0.2 to 0.8 g in⁻³.

The second washcoat region, the second washcoat zone or the secondwashcoat layer may or may not comprise an alkaline earth metal.

Typically, the alkaline earth metal comprises magnesium (Mg), calcium(Ca), strontium (Sr), barium (Ba) or a combination of two or morethereof. It is preferred that the alkaline earth metal comprises calcium(Ca), strontium (Sr), or barium (Ba), more preferably strontium (Sr) orbarium (Ba), and most preferably the alkaline earth metal comprisesbarium (Ba).

Typically, the amount of the alkaline earth metal is 0.07 to 3.75 molft⁻³, particularly 0.1 to 3.0 mol ft⁻³, more particularly 0.2 to 2.5 molft⁻³ (e.g. 0.25 to 1.0 mol ft⁻³), such as 0.3 to 2.25 mol ft⁻³,especially 0. 0.35 to 1.85 mol ft⁻³, preferably 0.4 to 1.5 mol ft⁻³,more preferably 0.5 to 1.25 mol ft⁻³.

When the second washcoat region, the second washcoat zone or the secondwashcoat layer further comprises an alkaline earth metal, then thesecond support material may or may not comprise, or consist essentiallyof, alumina doped with a heteroatom component. The heteroatom componenttypically comprises silicon, magnesium, barium, lanthanum, cerium,titanium, or zirconium or a combination of two or more thereof. Theheteroatom component may comprises, or consist essentially of, an oxideof silicon, an oxide of magnesium, an oxide of barium, an oxide oflanthanum, an oxide of cerium, an oxide of titanium or an oxide ofzirconium. More preferably, the alumina doped with a heteroatomcomponent is alumina doped with silica or alumina doped with magnesiumoxide. Even more preferably, the alumina doped with a heteroatomcomponent is alumina doped with silica.

The second washcoat region, the second washcoat zone or the secondwashcoat layer may consist essentially of a second PGM, a second supportmaterial and an alkaline earth metal component. The second washcoatregion, the second washcoat zone or the second washcoat layer preferablyconsists essentially of a second PGM and a second support material.

In general, the third washcoat region is directly disposed (i.e.directly supported) on the substrate. This means that the third washcoatregion is in contact with a surface of the substrate. It is preferredthat there is no intermediary washcoat region between a surface of thesubstrate and the third washcoat region.

The third washcoat region is typically downstream of the first washcoatregion (i.e. the third washcoat region is nearer to an outlet end of thesubstrate than the first washcoat region is to the inlet end). Exhaustgas entering the oxidation catalyst will come into contact with both thefirst washcoat region (and also the second washcoat region) before itcomes into contact with the third washcoat region.

Exhaust gas entering the catalyst generally comes into contact with thethird washcoat region after it has contacted the first and secondwashcoat regions. The first and second washcoat regions advantageouslygenerate heat by oxidising HCs and CO, which can bring the temperatureof the exhaust gas and the third washcoat region up to its light offtemperature for NO oxidation.

Generally, the third washcoat region has a different composition (i.e. adifferent overall composition) to the second washcoat region. It ispreferred that the third washcoat region has a different composition(i.e. a different overall composition) to both the first washcoat regionand the second washcoat region.

The third washcoat region may comprise, or consist of, a third washcoatlayer.

The third washcoat region may comprise, or consist of, a third washcoatzone. The third washcoat zone may comprise, or consist of, a thirdwashcoat layer. Thus, the third washcoat zone comprises, or consists of,the whole or complete length of the third washcoat layer.

The third washcoat region may comprise, or consist of, a part or portionof the third washcoat layer (see (3) and (7) in FIGS. 3 and 4). The partof the third washcoat layer is a part length (e.g. not the whole orcomplete length) of the third washcoat layer. When the third washcoatregion comprises, or consists of, a part or portion of the thirdwashcoat layer, then typically there is an overlap between the firstwashcoat layer and the third washcoat layer.

The third washcoat region comprises a third platinum group metal (PGM).It is preferred that the only PGM in the third washcoat region is thethird PGM.

Typically, the third PGM is selected from the group consisting ofplatinum, palladium and a combination of platinum and palladium. Thethird PGM may be platinum (e.g. platinum only). The third PGM may bepalladium (e.g. palladium only). The third PGM may be a combination ofplatinum and palladium (e.g. platinum and palladium only).

The third washcoat region typically has a total loading of the third PGMof 5 to 300 g ft⁻³. Preferably, the total loading of the third PGM is7.5 to 250 g ft⁻³, more preferably 10 to 200 g ft⁻³ (e.g. 15 to 100 gft⁻³), still more preferably 15 to 150 g ft⁻³ (e.g. 20 to 75 g ft⁻³) andeven more preferably 25 to 100 g ft⁻³. When the third washcoat regioncomprises, or consists of, a third washcoat layer or a third washcoatzone, then preferably the third washcoat layer or third washcoat zonehas a total loading of the third PGM of 10 to 250 g ft⁻³, morepreferably 15 to 200 g ft⁻³ (e.g. 15 to 100 g ft⁻³), still morepreferably 20 to 150 g ft⁻³ (e.g. 20 to 75 g ft⁻³) and even morepreferably 25 to 100 g ft⁻³.

Generally, when the third PGM is a combination of platinum andpalladium, the third washcoat region, the third washcoat zone or thethird washcoat layer comprises a ratio of the total mass of platinum tothe total mass of palladium of 10:1 to 1:10 (e.g. 7.5:1 to 1:7.5),preferably 5:1 to 1:5 (e.g. 3:1 to 1:3), and more preferably 2.5:1 to1:2.5 (e.g. 2:1 to 1:2).

It is preferred that when the third PGM is a combination of platinum andpalladium then the third washcoat region, the third washcoat zone or thethird washcoat layer comprises a total mass of platinum that is greaterthan or equal to the total mass of palladium (e.g. the ratio of thetotal mass of platinum to the total mass of palladium is ≥1:1). Morepreferably, the ratio of the total mass of platinum to the total mass ofpalladium is 10:1 to 1:1 (e.g. 7.5:1 to 1.1:1), preferably 5:1 to1.25:1, and more preferably 2.5:1 to 1.5:1.

When the third PGM is palladium or a combination of platinum andpalladium, then the third washcoat region, the third washcoat zone orthe third washcoat layer may comprise gold. The third washcoat region,the third washcoat zone or the third washcoat zone may comprise apalladium-gold alloy (e.g. the palladium of the second PGM may be analloy with gold).

When the third washcoat region, the third washcoat zone or the thirdwashcoat layer comprises gold, such as a palladium-gold alloy, thengenerally the third washcoat region, the third washcoat zone or thethird washcoat layer comprises a ratio of the total mass of palladium(Pd) to the total mass of gold (Au) of 9:1 to 1:9, preferably 5:1 to1:5, and more preferably 2:1 to 1:2.

Typically, the third support material comprises, or consists essentiallyof, a refractory metal oxide. The refractory metal oxide may be selectedfrom the group consisting of alumina, silica, titania, zirconia, ceriaand mixed or composite oxides of two or more thereof. It is preferredthat the refractory metal oxide is selected from alumina, silica andmixed or composite oxides thereof. More preferably, the refractory metaloxide is selected from alumina and silica-alumina. Even more preferably,the refractory metal oxide is alumina.

When the third PGM is palladium or a combination of platinum andpalladium, then the third support material may or may not comprise, orconsist of, cerium oxide, particularly ceria (CeO₂) or ceria-zirconia(CeO₂—ZrO₂).

In general, the third PGM is disposed or supported on the third supportmaterial. When the third washcoat region, the third washcoat zone or thethird washcoat layer comprises gold, then the gold or the palladium-goldalloy may be disposed or supported on the third support material. Forexample, the third PGM and/or gold can be dispersed on the third supportmaterial and/or impregnated into the third support material.

The third washcoat region, the third washcoat layer or the thirdwashcoat zone typically comprises a total washcoat loading of 0.25 to 5g in⁻³, preferably 0.5 to 4 g in⁻³, such as 0.75 to 3 g in⁻³, morepreferably 1.0 to 2.5 g in⁻³, such as 1.25 to 2.0 g in⁻³.

The third washcoat region, the third washcoat zone or the third washcoatlayer may further comprise a hydrocarbon adsorbent. The hydrocarbonadsorbent may be selected from a zeolite, active charcoal, porousgraphite and a combination of two or more thereof. It is preferred thatthe hydrocarbon adsorbent is a zeolite.

When the third washcoat region, the third washcoat zone or the thirdwashcoat layer comprises a hydrocarbon adsorbent, then typically thetotal amount of hydrocarbon adsorbent is 0.05 to 3.00 g in⁻³,particularly 0.10 to 2.00 g in⁻³, more particularly 0.2 to 0.8 g in⁻³.

The third washcoat region, the third washcoat zone or the third washcoatlayer may or may not comprise an alkaline earth metal.

Typically, the alkaline earth metal comprises magnesium (Mg), calcium(Ca), strontium (Sr), barium (Ba) or a combination of two or morethereof. It is preferred that the alkaline earth metal comprises calcium(Ca), strontium (Sr), or barium (Ba), more preferably strontium (Sr) orbarium (Ba), and most preferably the alkaline earth metal comprisesbarium (Ba).

Typically, the amount of the alkaline earth metal is 0.07 to 3.75 molft⁻³, particularly 0.1 to 3.0 mol ft⁻³, more particularly 0.2 to 2.5 molft⁻³ (e.g. 0.25 to 1.0 mol ft⁻³), such as 0.3 to 2.25 mol ft⁻³,especially 0. 0.35 to 1.85 mol ft⁻³, preferably 0.4 to 1.5 mol ft⁻³,more preferably 0.5 to 1.25 mol ft⁻³.

When the third washcoat region, the third washcoat zone or the thirdwashcoat layer further comprises an alkaline earth metal, then the firstsupport material may or may not comprise, or consist essentially of,alumina doped with a heteroatom component. The heteroatom componenttypically comprises silicon, magnesium, barium, lanthanum, cerium,titanium, or zirconium or a combination of two or more thereof. Theheteroatom component may comprises, or consist essentially of, an oxideof silicon, an oxide of magnesium, an oxide of barium, an oxide oflanthanum, an oxide of cerium, an oxide of titanium or an oxide ofzirconium. More preferably, the alumina doped with a heteroatomcomponent is alumina doped with silica or alumina doped with magnesiumoxide. Even more preferably, the alumina doped with a heteroatomcomponent is alumina doped with silica.

The third washcoat region, the third washcoat zone or the third washcoatlayer may consist essentially of a third PGM, a third support materialand an alkaline earth metal component. The third washcoat region, thethird washcoat zone or the third washcoat layer preferably consistsessentially of a third PGM and a third support material.

A first aspect of the oxidation catalyst of the invention relates to anarrangement where (i) the third washcoat region is adjacent (e.g.directly adjacent) to the second washcoat region. The third washcoatregion may abut or be contiguous with the second washcoat region.Typically, the third washcoat region is adjacent to the second washcoatregion on the substrate (e.g. the third washcoat region is disposed nextto the first second region along the length of the substrate).

In the first oxidation catalyst aspect, typically each of the secondwashcoat region and the third washcoat region is directly disposed (i.e.directly supported) on the substrate.

The second washcoat region, in the first oxidation catalyst aspect, maybe disposed at or adjacent to an inlet end or an outlet end of thesubstrate.

The third washcoat region is typically disposed at or adjacent to anoutlet end of the substrate.

Generally, in the first oxidation catalyst aspect, the total loading ofthe second PGM (e.g. in the second washcoat region or second washcoatzone) is greater than the total loading of the third PGM (e.g. in thethird washcoat region or the third washcoat zone).

In the first oxidation catalyst aspect, the total loading of the secondPGM (e.g. in the second washcoat region or the second washcoat zone) istypically greater than the total loading of the first PGM (e.g. in thefirst washcoat region or the first washcoat zone).

Typically, in the first oxidation catalyst aspect, the first washcoatregion comprises a part or portion (e.g. a front part or portion) of anupstream washcoat layer (e.g. the first washcoat layer as definedherein), the second washcoat region comprises a part or portion (e.g. arear part or portion) of the upstream washcoat layer and a part orportion (e.g. a front part or portion) of a downstream washcoat layer(e.g. the third washcoat layer as defined herein), and the thirdwashcoat region comprises a part or portion (e.g. a rear part orportion) of the downstream washcoat layer. This first oxidation catalystaspect of the invention embraces two arrangements of the catalyst, whichdepend on the nature of the overlap in the second washcoat region. Sucharrangements can be obtained by using a method in accordance with thesecond method aspect of the invention.

Generally, in the first oxidation catalyst aspect, the sum of thelengths of the first washcoat region, the second washcoat region and thethird washcoat region is preferably the same as the length of thesubstrate.

The second washcoat region may have a length as defined above.Preferably the second washcoat region has a length of 10 to 40% of thelength of the substrate, more preferably 15 to 35% of the length of thesubstrate, such as 20 to 30% of the length of the substrate.

The third washcoat region, in the first oxidation catalyst aspect,typically has a length of 10 to 40% of the length of the substrate, morepreferably 15 to 35% of the length of the substrate, such as 20 to 30%of the length of the substrate.

In the first oxidation catalyst aspect, when the first washcoat regioncomprises, or consists of, a part or portion of an upstream washcoatlayer (e.g. the first washcoat layer as defined herein), then the firstwashcoat region or the part or portion of the upstream washcoat layermay comprise a PGM zone. The PGM may be as defined above.

In a first arrangement, the second washcoat region may comprise, orconsist of, a rear part or portion of the upstream washcoat layerdisposed on (i.e. overlapping) a front part or portion of the downstreamwashcoat layer (see FIG. 3). The downstream washcoat layer (e.g. all orthe whole of the downstream washcoat layer) is typically directlydisposed on the substrate. It is preferred that a rear part or portionof the downstream washcoat layer does not support (e.g. is not coveredby or does not underlie) the upstream washcoat layer. In the secondmethod aspect, the first washcoat of length L₁ (see (7) in FIG. 3) formspart of the third washcoat region and the part of second washcoat regiondirectly disposed on the substrate. The second washcoat of length L₂(see (6) in FIG. 3) forms part of the first washcoat region and the partof the second washcoat region that is disposed on an underlying washcoatlayer. The length of the first washcoat region is given by L−L₁, thelength of the second washcoat region is given by L₂+L₁−L, and the lengthof the third washcoat region is given by L−L₂.

In the first arrangement, the first PGM (e.g. of the first washcoatregion) is typically platinum (e.g. platinum only). The third PGM (e.g.of the third washcoat region) is preferably a combination of platinumand palladium. The second PGM (e.g. of the second washcoat region) is acombination of platinum and palladium. The second washcoat regioncomprises part of the first PGM from the first washcoat layer and partof the third PGM from the downstream washcoat layer (e.g. the thirdwashcoat layer).

Alternatively, in a second arrangement, the second washcoat region maycomprise, or consist of, a front part or portion of the downstreamwashcoat layer disposed on (i.e. overlapping) a rear part or portion ofthe upstream washcoat layer (see FIG. 4). The upstream washcoat layer(e.g. all or the whole of the upstream washcoat layer) is typicallydirectly disposed on the substrate. It is preferred that a front part orportion of the upstream washcoat layer does not support (e.g. is notcovered by or does not underlie) the downstream washcoat layer. In thesecond method aspect, the first washcoat of length L₁ (see (6) in FIG.4) forms part of the first washcoat region and the part of secondwashcoat region directly disposed on the substrate. The second washcoatof length L₂ (see (7) in FIG. 4) forms part of the third washcoat regionand the part of the second washcoat region that is disposed on anunderlying washcoat layer. The length of the first washcoat region isgiven by L−L₂, the length of the second washcoat region is given byL₂+L₁−L, and the length of the third washcoat region is given by L−L₁.

In the second arrangement, the first PGM (e.g. of the first washcoatregion) is typically a combination of platinum and palladium. The thirdPGM (e.g. of the third washcoat region) is preferably platinum (e.g.platinum only). The second PGM (e.g. of the second washcoat region) is acombination of platinum and palladium.

In the second arrangement, it is preferred that the first washcoatregion or the upstream washcoat layer comprises a total mass of platinumthat is greater than or equal to the total mass of palladium, such asdescribed above.

The first aspect of the oxidation catalyst of the invention also relatesto an oxidation catalyst where the first washcoat region comprises afirst washcoat zone, the second washcoat region comprises a secondwashcoat zone, and the third washcoat region comprises a third washcoatzone. It is preferred that the first washcoat region consists of a firstwashcoat zone, the second washcoat region consists of a second washcoatzone and the third washcoat region consists of a third washcoat zone.

The second washcoat zone preferably has a length that is greater than orequal to the length of the first washcoat zone. More preferably, thesecond washcoat zone has a length that is greater than the length of thefirst washcoat zone. Even more preferably, the second washcoat zone hasa length that is greater than the lengths of each of the first washcoatzone and the third washcoat zone.

The first washcoat zone typically consists of a first washcoat layer. Itis preferred that there is no overlap between the first washcoat layerand the second washcoat layer. The first PGM of the first washcoat zoneis as defined above.

The second washcoat zone may comprise, or consist of, a single washcoatlayer, such as the second washcoat layer (as defined herein). The secondwashcoat zone may comprise, or consist of, two washcoat layers, such asthe second washcoat layer (as defined herein) and a fourth washcoatlayer, wherein the second washcoat layer is disposed on the fourthwashcoat layer.

It is preferred that there is no overlap between the second washcoatlayer and the third washcoat layer. When the second washcoat zonecomprises a fourth washcoat layer, then it is preferred that there is nooverlap between the fourth washcoat layer and the third washcoat layer.

In the first oxidation catalyst aspect, when the second washcoat regioncomprises, or consists of, a second washcoat zone, then preferably thesecond PGM is a combination of platinum and palladium. When the secondwashcoat zone comprises, or consists of, a second washcoat layer and afourth washcoat layer, then preferably the second washcoat layercomprises platinum (e.g. platinum is the only PGM in the second washcoatlayer) and the fourth washcoat layer comprises a combination of platinumand palladium (e.g. platinum and palladium are the only PGMs in thefourth washcoat layer).

The third washcoat zone may consist of a third washcoat layer.

In the first oxidation catalyst aspect, when the third washcoat regioncomprises, or consists of, a third washcoat zone or a third washcoatlayer, then the third PGM is preferably platinum (e.g. platinum only).

An oxidation catalyst of the first aspect comprising a first washcoatzone, a second washcoat zone and a third washcoat zone can be made usinga method of the first method aspect of the invention. The first washcoatzone or the third washcoat zone can be formed by impregnation step (iv)of the method.

The PGM zone or the first washcoat zone may be formed by an impregnationstep in accordance with the method of the invention. When the PGM zoneor the first washcoat zone is formed by the impregnation step, the PGMzone or the first washcoat zone has a length represented by L₃. If thefirst washcoat is impregnated, then preferably L₃<L₁. The secondwashcoat zone then has a length represented by L₁−L₃. The secondwashcoat zone is formed by the remaining, non-impregnated part of thefirst washcoat. If the second washcoat is impregnated, then preferablyL₃<L₂. The second washcoat zone then has a length represented by L₂−L₃.The second washcoat zone is formed by the remaining, non-impregnatedpart of the second washcoat.

A second aspect of the oxidation catalyst of the invention relates to anarrangement where (ii) the second washcoat region is disposed orsupported on the third washcoat region. Typically, the third washcoatregion is adjacent to the first washcoat region. The third washcoatregion is adjacent to the first washcoat region on the substrate (e.g.the third washcoat region is disposed next to the first washcoat regionalong the length of the substrate). It is preferred that both the secondwashcoat region and the third washcoat region are adjacent to the firstwashcoat region. Thus, both the second washcoat region and the thirdwashcoat region are disposed next to the first washcoat region along thelength of the substrate.

The second washcoat region and/or the third washcoat region is/aretypically disposed at or adjacent to an outlet end of the substrate.

In the second oxidation catalyst aspect of the invention, the firstwashcoat region comprises, or consists of, a first washcoat layer or afirst washcoat zone, the second washcoat region comprises a secondwashcoat layer, and the third washcoat region comprises a third washcoatlayer. It is preferred that the first washcoat region consists of afirst washcoat layer or a first washcoat zone, the second washcoatregion consists of a second washcoat layer and the third washcoat regionconsists of a third washcoat layer. More preferably, the first washcoatregion consists of a first washcoat layer.

When the first washcoat region comprises, or consists of, a firstwashcoat zone, then the first washcoat zone may be a PGM zone as definedabove.

It is preferred that there is no overlap between the first washcoatlayer and (a) the second washcoat layer and/or (b) the third washcoatlayer.

Typically, in the second oxidation catalyst aspect of the invention, thesecond PGM is platinum (e.g. platinum only) and the third PGM isselected from the group consisting of palladium (e.g. palladium only)and a combination of platinum and palladium (e.g. platinum and palladiumonly). It is preferred that the third PGM is a combination of platinumand palladium. The first PGM is as defined above.

When the third PGM is a combination of platinum and palladium, the thirdwashcoat layer comprises a ratio of the total mass of platinum to thetotal mass of palladium of 10:1 to 1:10 (e.g. 7.5:1 to 1:7.5),preferably 5:1 to 1:5 (e.g. 3:1 to 1:3), and more preferably 2.5:1 to1:2.5 (e.g. 2:1 to 1:2), such as about 7:6 to 6:7.

In the second oxidation catalyst aspect, the length of the thirdwashcoat layer may be equal to, greater than or less than the length ofthe second washcoat layer. It is preferred that the length of the thirdwashcoat layer is about the same as the length of the second washcoatlayer.

Generally, in the second oxidation catalyst aspect, the length of thesecond washcoat layer is from 20 to 90% of the length of the substrate,more preferably 30 to 80% of the length of the substrate, such as 40 to70% of the length of the substrate, and even more preferably 40 to 60%of the length of the substrate.

It is preferred in the second oxidation catalyst aspect of the inventionthat the second washcoat layer has a length of greater than or equal to50% of the length of the substrate. The second washcoat layer preferablyhas a length of 50 to 95% of the length of the substrate, morepreferably 60 to 92.5% of the length of the substrate, such as 70 to 90%of the length of the substrate, and even more preferably 75 to 85% ofthe length of the substrate.

When the second washcoat layer has a length greater than or equal to 50%of the length of the substrate in the second oxidation catalyst aspectof the invention, then preferably the first washcoat region, the firstwashcoat zone or the first washcoat layer has a length of 5 to 50% ofthe length of the substrate, more preferably 7.5 to 40% of the length ofthe substrate, particularly 10 to 30% of the length of the substrate,and even more preferably 15 to 25% of the length of the substrate.

The second washcoat layer may overlap an end of the third washcoat layersuch that a part or portion of the second washcoat layer is in directcontact with the substrate. Alternatively, the second washcoat layer maybe disposed entirely on the third washcoat layer.

The third washcoat layer preferably has a length of greater than orequal to 50% of the length of the substrate. The third washcoat layerpreferably has a length of 50 to 95% of the length of the substrate,more preferably 60 to 92.5% of the length of the substrate, such as 70to 90% of the length of the substrate, and even more preferably 75 to 85of the length of the substrate.

In the second oxidation catalyst aspect, the total loading of the secondPGM is generally greater than the total loading of the third PGM.However, when the first washcoat zone comprises, or consists of, a firstwashcoat layer and a fourth washcoat layer, then alternatively the totalloading of the third PGM may be greater than or equal to, preferablygreater than, the total loading of the second PGM. It is preferred thatthe total loading of the second PGM is greater than the total loading ofthe third PGM, even when the first washcoat zone comprises, or consistsof, a first washcoat layer and a fourth washcoat layer.

When the first washcoat region comprises, or consists of, a firstwashcoat zone, then the first washcoat zone may comprise, or consist of,a first washcoat layer and a fourth washcoat layer, wherein the fourthwashcoat layer is disposed on the first washcoat layer, such asdescribed above. The first washcoat layer and the fourth washcoat layertypically have about the same length. Such an oxidation catalyst may beprepared according to the second method aspect of the invention.

In the second oxidation catalyst aspect, when the first washcoat zonecomprises, or consists of, a first washcoat layer and a fourth washcoatlayer, then typically the total loading of the first PGM (e.g. of thefirst washcoat zone) is greater than the total loading of each of thesecond PGM (e.g. of the second washcoat layer) and the third PGM (e.g.of the third washcoat layer).

Each refractory metal (e.g. of the first, second or third supportmaterials) oxide may be doped. The inclusion of a dopant can thermallystabilise the first support material. It is to be understood that anyreference to “doped” in this context refers to a material where the bulkor host lattice of the refractory metal oxide is substitution doped orinterstitially doped with a dopant.

When the refractory metal oxide is doped, then the total amount ofdopant is 0.1 to 5 by weight (i.e. % by weight of the refractory metaloxide). It is preferred that the total amount of dopant is 0.25 to 2.5%by weight, more preferably 0.5 to 1.5% by weight (e.g. about 1% byweight). The refractory metal oxide may be doped with one or more dopantselected from the group consisting of zirconium (Zr), titanium (Ti),silicon (Si), yttrium (Y), lanthanum (La), cerium (Ce), praseodymium(Pr), samarium (Sm), neodymium (Nd) and an oxide thereof.

When the hydrocarbon adsorbent is a zeolite, then preferably eachzeolite is a medium pore zeolite (e.g. a zeolite having a maximum ringsize of eight tetrahedral atoms) or a large pore zeolite (e.g. a zeolitehaving a maximum ring size of ten tetrahedral atoms).

Examples of suitable zeolites or types of zeolite include faujasite,clinoptilolite, mordenite, silicalite, ferrierite, zeolite X, zeolite Y,ultrastable zeolite Y, AEI zeolite, ZSM-5 zeolite, ZSM-12 zeolite,ZSM-20 zeolite, ZSM-34 zeolite, CHA zeolite, SSZ-3 zeolite, SAPO-5zeolite, offretite, a beta zeolite or a copper CHA zeolite. Each zeoliteis preferably ZSM-5, a beta zeolite or a Y zeolite.

Typically, the first washcoat region, first washcoat zone or firstwashcoat layer does not comprise rhodium.

It may be preferable that the second washcoat region, the secondwashcoat zone or the second washcoat layer does not comprise rhodium.

The third washcoat region, the third washcoat zone or the third washcoatlayer may not comprise rhodium.

In general, the oxidation catalyst of the invention may or may notcomprise rhodium. It is preferred that the oxidation catalyst does notcomprise ruthenium, rhodium, and iridium.

The oxidation catalyst of the invention is, in general, not a three-waycatalyst (i.e. it does not have three-way catalyst activity). Inparticular, the oxidation catalyst of the invention does not typicallyperform substantial reduction of NO_(x) to N₂ and O₂.

A further general feature of the oxidation catalyst of the invention isthat when an alkali metal, particularly sodium or potassium, andespecially potassium, is present, then preferably only the hydrocarbonadsorbent comprises the alkali metal, especially when the hydrocarbonadsorbent is a zeolite. It is further preferred that the oxidationcatalyst of the invention does not comprise an alkali metal,particularly sodium or potassium.

The oxidation catalyst of the invention may or may not comprise analkaline earth metal component (as defined above) and/or alumina dopedwith a heteroatom component (e.g. a heteroatom component as definedabove).

Substrates for supporting oxidation catalysts for treating the exhaustgas of a diesel are well known in the art. The substrate typically has aplurality of channels (e.g. for the exhaust gas to flow through).Generally, the substrate is a ceramic material or a metallic material.

It is preferred that the substrate is made or composed of cordierite(SiO₂—Al₂O₃—MgO), silicon carbide (SiC), Fe—Cr—Al alloy, Ni—Cr—Al alloy,or a stainless steel alloy.

The substrate may have a diameter of from 2.5 to 15 inches, such as 4 to15 inches, preferably 5 to 12.5 inches, such as 6 to 10 inches.

The length of the substrate is typically 2.5 to 15 inches, such as 3 to12.5 inches, preferably 4 to 11 inches (e.g. 5 to 10 inches).

Typically, the substrate is a monolith (also referred to herein as asubstrate monolith). Such monoliths are well-known in the art. Thesubstrate monolith may be a flow-through monolith or a filteringmonolith.

A flow-through monolith typically comprises a honeycomb monolith (e.g. ametal or ceramic honeycomb monolith) having a plurality of channelsextending therethrough, which channels are open at both ends. When thesubstrate is a flow-through monolith, then the oxidation catalyst of theinvention is typically a diesel oxidation catalyst (DOC) or is for useas a diesel oxidation catalyst (DOC).

A filtering monolith generally comprises a plurality of inlet channelsand a plurality of outlet channels, wherein the inlet channels are openat an upstream end (i.e. exhaust gas inlet side) and are plugged orsealed at a downstream end (i.e. exhaust gas outlet side), the outletchannels are plugged or sealed at an upstream end and are open at adownstream end, and wherein each inlet channel is separated from anoutlet channel by a porous structure. When the substrate is a filteringmonolith, then the oxidation catalyst of the invention is typically acatalysed soot filter (CSF) or is for use as a catalysed soot filter(CSF).

When the monolith is a filtering monolith, it is preferred that thefiltering monolith is a wall-flow filter. In a wall-flow filter, eachinlet channel is alternately separated from an outlet channel by a wallof the porous structure and vice versa. It is preferred that the inletchannels and the outlet channels are arranged in a honeycombarrangement. When there is a honeycomb arrangement, it is preferred thatthe channels vertically and laterally adjacent to an inlet channel areplugged at an upstream end and vice versa (i.e. the channels verticallyand laterally adjacent to an outlet channel are plugged at a downstreamend). When viewed from either end, the alternately plugged and open endsof the channels take on the appearance of a chessboard.

In principle, the substrate may be of any shape or size. However, theshape and size of the substrate is usually selected to optimise exposureof the catalytically active materials in the catalyst to the exhaustgas. The substrate may, for example, have a tubular, fibrous orparticulate form. Examples of suitable supporting substrates include asubstrate of the monolithic honeycomb cordierite type, a substrate ofthe monolithic honeycomb SiC type, a substrate of the layered fibre orknitted fabric type, a substrate of the foam type, a substrate of thecrossflow type, a substrate of the metal wire mesh type, a substrate ofthe metal porous body type and a substrate of the ceramic particle type.

In general, the oxidation catalyst of the invention is for use as adiesel oxidation catalyst (DOC).

The invention also provides an exhaust system comprising the oxidationcatalyst and an emissions control device. In general, the emissionscontrol device is separate to the oxidation catalyst (e.g. the emissionscontrol device has a separate substrate to the substrate of theoxidation catalyst), and preferably the oxidation catalyst is upstreamof the emissions control device.

The emissions control device may be selected from a diesel particulatefilter (DPF), a NO_(x) adsorber catalyst (NAC), a lean NO_(x) catalyst(LNC), a selective catalytic reduction (SCR) catalyst, a dieseloxidation catalyst (DOC), a catalysed soot filter (CSF), a selectivecatalytic reduction filter (SCRF™) catalyst, and combinations of two ormore thereof. Emissions control devices represented by the terms dieselparticulate filters (DPFs), NO_(x) adsorber catalysts (NACs), leanNO_(x) catalysts (LNCs), selective catalytic reduction (SCR) catalysts,diesel oxidation catalyst (DOCs), catalysed soot filters (CSFs) andselective catalytic reduction filter (SCRF™) catalysts are all wellknown in the art.

An emissions control device having a filtering substrate may be selectedfrom the group consisting of a diesel particulate filter (DPF), acatalysed soot filter (CSF) and a selective catalytic reduction filter(SCRF™) catalyst.

In a first exhaust system embodiment, the exhaust system comprises theoxidation catalyst of the invention (e.g. as a DOC) and either a dieselparticulate filter (DPF) or a catalysed soot filter (CSF). Thisembodiment also relates to the use of the oxidation catalyst fortreating an exhaust gas from a diesel engine in combination with adiesel particulate filter or a catalysed soot filter. The oxidationcatalyst is typically followed by (e.g. is upstream of) the dieselparticulate filter (DPF) or the catalysed soot filter (CSF). Thus, forexample, an outlet of the oxidation catalyst is connected to an inlet ofthe diesel particulate filter or the catalysed soot filter.

In a second exhaust system embodiment, the exhaust system comprises theoxidation catalyst of the invention (e.g. as a DOC or a NAC, preferablya DOC) and a selective catalytic reduction (SCR) catalyst. Thisembodiment also relates to the use of the oxidation catalyst fortreating an exhaust gas from a diesel engine in combination with aselective catalytic reduction filter (SCRF™) catalyst. The oxidationcatalyst of the invention is typically followed by (e.g. is upstream of)the selective catalytic reduction (SCR) catalyst.

A third exhaust system embodiment comprises the oxidation catalyst ofthe invention (e.g. as a DOC) and a selective catalytic reduction filter(SCRF™) catalyst. This embodiment also relates to the use of theoxidation catalyst for treating an exhaust gas from a diesel engine incombination with a selective catalytic reduction filter (SCRF™)catalyst. The oxidation catalyst of the invention is typically followedby (e.g. is upstream of) the selective catalytic reduction filter(SCRF™) catalyst

A fourth exhaust system embodiment relates to an exhaust systemcomprising the oxidation catalyst of the invention (e.g. as a DOC), adiesel particulate filter or a catalysed soot filter (CSF), and aselective catalytic reduction (SCR) catalyst. The DOC/DPF/SCR orDOC/CSF/SCR arrangement is a preferred exhaust system for a light-dutydiesel vehicle. This embodiment also relates to the use of the oxidationcatalyst for treating an exhaust gas from a diesel engine in combinationwith either a diesel particulate filter or a catalysed soot filter(CSF), and a selective catalytic reduction (SCR) catalyst. The oxidationcatalyst is typically followed by (e.g. is upstream of) the dieselparticulate filter or the catalysed soot filter (CSF). The DPF or CSF istypically followed by (e.g. is upstream of) the selective catalyticreduction (SCR) catalyst.

In a fifth exhaust system embodiment, the exhaust system comprises theoxidation catalyst of the invention (e.g. as a DOC), a selectivecatalytic reduction (SCR) catalyst and either a catalysed soot filter(CSF) or a diesel particulate filter (DPF). This embodiment also relatesto the use of the oxidation catalyst for treating an exhaust gas from adiesel engine in combination with a selective catalytic reduction (SCR)catalyst and either a catalysed soot filter (CSF) or a dieselparticulate filter (DPF). The oxidation catalyst of the invention istypically followed by (e.g. is upstream of) the selective catalyticreduction (SCR) catalyst. The selective catalytic reduction (SCR)catalyst is typically followed by (e.g. are upstream of) the catalysedsoot filter (CSF) or the diesel particulate filter (DPF).

When the exhaust system comprises a DPF or a CSF, then the exhaustsystem may comprise a means for heating the DPF or the CSF to aregeneration temperature. Additional heat may be required for activeregeneration of the DPF or CSF. The means for heating the DPF or the CSFto a regeneration temperature may be selected from resistive heatingcoils, an injector for injecting combustible HC into the exhaust gasdownstream of the diesel engine and means for adjusting engine operationto generate additional HC in the exhaust gas. It is preferred that themeans for heating the DPF or the CSF to a regeneration temperature is ameans for adjusting engine operation to generate additional HC in theexhaust gas.

The DPF or CSF may further comprise resistive heating coils. When theexhaust system comprises an injector for injecting combustible HC intothe exhaust gas, then the injector is downstream of the diesel engineand upstream the DPF or CSF and optionally upstream of the oxidationcatalyst.

A nitrogenous reductant injector may be directly upstream of theselective catalytic reduction (SCR) catalyst or the selective catalystreduction filter (SCRF™) catalyst. Thus, in the second, third and fifthexhaust system embodiments, the oxidation catalyst may be followed by(e.g. is upstream of) the nitrogenous reductant injector, and thenitrogenous reductant injector may be followed by (e.g. is upstream of)the SCR catalyst or the SCRF™ catalyst. In the fourth exhaust systemembodiment, the DPF or CSF may be followed by (e.g. is upstream of) anitrogenous reductant injector, and the nitrogenous reductant injectormay be followed by (e.g. is upstream of) the selective catalyticreduction (SCR) catalyst.

The exhaust system may further comprise an NO₂ sensor and/or atemperature sensor.

When the exhaust system comprises an SCR catalyst or an SCRF™ catalyst,then it is preferred that the SCR catalyst or the SCRF™ catalystcomprises a copper exchanged zeolite or an iron exchanged zeolite. It ispreferred that the SCR catalyst or the SCRF™ catalyst comprises a copperexchanged zeolite, such as a copper exchanged zeolite where the zeolitehas a chabazite (CHA) structure.

The invention further provides a vehicle comprising a diesel engine andeither an exhaust system of the invention or an oxidation catalyst ofthe invention. Typically, the oxidation catalyst is located downstreamof the diesel engine, such as downstream of a turbo of the dieselengine.

The diesel engine may be a homogeneous charge compression ignition(HCCI) engine, a pre-mixed charge compression ignition (PCCI) engine ora low temperature combustion (LTC) engine. It is preferred that thediesel engine is a conventional (i.e. traditional) diesel engine.

The vehicle may be a light-duty diesel vehicle (LDV), such as defined inUS or European legislation. A light-duty diesel vehicle typically has aweight of <2840 kg, more preferably a weight of <2610 kg.

In the US, a light-duty diesel vehicle (LDV) refers to a diesel vehiclehaving a gross weight of ≤8,500 pounds (US lbs). In Europe, the termlight-duty diesel vehicle (LDV) refers to (i) passenger vehiclescomprising no more than eight seats in addition to the driver's seat andhaving a maximum mass not exceeding 5 tonnes, and (ii) vehicles for thecarriage of goods having a maximum mass not exceeding 12 tonnes.

Alternatively, the vehicle may be a heavy-duty diesel vehicle (HDV),such as a diesel vehicle having a gross weight of >8,500 pounds (USlbs), as defined in US legislation.

Methods for preparing the oxidation catalyst of the invention are knownin the art. See, for example, our WO 99/47260, WO 2007/077462 and WO2011/080525. Similarly, the conditions for drying and calcining awashcoat are also well known.

The first method aspect of producing an oxidation catalyst comprises thesteps of: (i) coating a substrate with a first washcoat of length L₁,wherein the substrate has an axial length L and L₁ is less than or equalto the axial length L (e.g. L₁≤L); then (ii) coating the substrate witha second washcoat of length L₂, wherein L₂ is less than or equal to theaxial length L (e.g. L₂≤L); (iii) drying the first washcoat and thesecond washcoat coated onto the substrate; (iv) impregnating at leastone of the first washcoat and the second washcoat with a platinum groupmetal to a length L₃, wherein L₃ is less than the axial length L (e.g.L₃<L); and (v) calcining the substrate coated with the first washcoat,the second washcoat and with the impregnated platinum group metal.Methods for impregnating a washcoat or layer with a PGM are known in theart (see, for example, WO 2013/088152). The step of impregnating with aPGM can be used to form a zone, such as a first washcoat zone or a PGMzone as defined above.

Typically, the length L₃ is either less than the length L₁ or is lessthan the length L₂. It is preferred that the length L₃ is less than thelength L₁ and the length L₃ is less than the length L₂.

In general, the first washcoat region or the third washcoat region ofthe oxidation catalyst is provided by step (iv) of impregnating at leastone of the first washcoat and the second washcoat with a platinum groupmetal to a length L₃. It is preferred that the first washcoat region ofthe oxidation catalyst is provided by step (iv).

Step (iv) may comprise (iv) impregnating at least one of the firstwashcoat and the second washcoat with a platinum group metal to a lengthL₃ from a first end or a second end of the substrate.

Step (i) of the method typically comprises (i) coating a substrate froma first end with a first washcoat of length L₁.

Step (ii) may comprise (ii) coating a substrate from the first end witha second washcoat of length L₂. Thus, the substrate is coated with thefirst washcoat and the second washcoat from the same end.

It is preferred that at least one of the length L₁ of the first washcoator the length L₂ of the second washcoat is equal to the axial length Lof substrate (e.g. L₁=L or L₂=L). More preferably, the length L₁ of thefirst washcoat is equal to the axial length L of substrate.

The length L₂ of the second washcoat may be less than or equal to thelength L₁ of the first washcoat (e.g. L₂≤L₁). It is preferred that thelength L₂ of the second washcoat is equal to the axial length L of thesubstrate (e.g. L₂=L).

When the substrate is coated with the first washcoat and the secondwashcoat from the same end (e.g. the first end), then preferably step(iii) comprises (iii) impregnating both the first washcoat and thesecond washcoat with a platinum group metal to a length L₃ from thefirst end of the substrate.

Alternatively, step (ii) may comprise, or consist of, (ii) coating asubstrate from a second end of the substrate with a second washcoat oflength L₂. It is preferred that L₂ is greater than the differencebetween L and L₁ (e.g. L₂>L−L₁).

The second method aspect of the method of producing an oxidationcatalyst of the invention is particularly advantageous because it iscost effective. In particular, the method allows the preparation of anoxidation catalyst of the invention using a single calcination pass.

In general, the method for preparing an oxidation catalyst of theinvention comprises a single step of calcining the substrate coated withthe first washcoat, the second washcoat and optionally the impregnatedplatinum group metal.

The second method aspect of producing the oxidation catalyst comprises,or consists of, the steps: (i) coating a substrate from a first end witha first washcoat of length L₁, wherein the substrate has an axial lengthL and L₁ is less than the axial length L (e.g. L₁<L); then (ii) coatingthe substrate from a second end with a second washcoat of length L₂,wherein L₂ is greater than the difference between L and L₁ (e.g.L₂>L−L₁); and (iii) calcining the substrate coated with the firstwashcoat and the second washcoat. The length L₂ is less than or equal tothe length of the substrate L (e.g. L₂≤L), preferably length L₂ is lessthan or equal to the length of the substrate (e.g. L₂<L).

This method aspect of the invention can be used to produce an oxidationcatalyst of the invention according to the first oxidation catalystaspect where the second washcoat region is formed by the overlap betweenan upstream washcoat layer and a downstream washcoat layer (e.g. a thirdwashcoat layer). Since the sum of the lengths of the first and secondwashcoats is greater than the axial length of the substrate (i.e.|L₂+L₁|>L), the second washcoat overlaps the first washcoat on thesubstrate to form the second washcoat region. The length of the secondwashcoat region L_(M) may be represented by L_(M)=L₂+L₁−L.

In this method aspect of the invention, the step of coating a substratefrom a first end with a first washcoat provides either (a) thedownstream washcoat layer (e.g. the third washcoat layer) in the firstarrangement of the first oxidation catalyst arrangement or (b) the firstwashcoat layer in the second arrangement of the first oxidation catalystarrangement.

The invention also provides a method of modulating the content ofnitrogen oxides (NO_(x)) in an exhaust gas from a diesel engine for anemissions control device. Any reference to “modulate the NO_(x) content”as used herein, particularly in relation to method or use aspects of theinvention, refers to changing (i.e. adjusting) or maintaining the ratio(in ppm or % volume, typically at the temperature and pressure of theexhaust gas) of NO:NO₂ to be within a predefined range.

In general, “modulate the NO_(x) content” refers to changing ormaintaining, preferably changing, the ratio (in ppm or % volume) ofNO:NO₂ in an exhaust gas, typically directly from the diesel engine, tobe less than 17:3 (i.e. the amount of NO to NO₂ is less than that whichis normally found in an exhaust gas from a diesel engine), preferablythe ratio of NO:NO₂ is from 5:1 to 1:5, more preferably 2.5:1 to 1:2.5,and even more preferably 2:1 to 1:2 (e.g. 1.5:1 to 1:1.5 or about 1:1).

Definitions

The acronym “PGM” as used herein refers to “platinum group metal”. Theterm “platinum group metal” generally refers to the metals Ru, Rh, Pd,Os, Ir and Pt of the Periodic Table, particularly the metals Ru, Rh, Pd,Ir and Pt.

The term “washcoat” is known in the art and refers to an adherentcoating that is applied to a substrate during production of a catalyst.The coating or washcoat generally comprises one or more components of acatalyst formulation.

The term “washcoat region” used herein refers to an area or portion ofone or more washcoats on a substrate. A washcoat region has a distinct,overall composition that is different to an adjacent or neighbouringwashcoat region. The term “washcoat region” embraces the terms “washcoatlayer” and “washcoat zone”. A washcoat region may comprise, or consistof, a single washcoat layer. The washcoat region may comprise, orconsist of, two washcoat layers. The washcoat region may comprise all orpart of each washcoat layer. For example, a washcoat region could be thearea or region of washcoats on a substrate where an end of a firstwashcoat layer overlaps with an end of a second washcoat layer (e.g. see(2) in FIGS. 3 and 4). A washcoat region may comprise, or consist of, asingle washcoat zone.

The term “washcoat zone” used herein refers to the horizontalarrangement of either one or more washcoats along the length of asubstrate or an impregnated part or portion of one or more washcoatsalong the length of a substrate. A “washcoat zone” has a distinctupstream and downstream boundary or edge (i.e. it is possible todistinguish one washcoat zone from another washcoat zone or layer usingconventional analytical techniques).

When the “washcoat zone” refers to the arrangement of one or morewashcoats along the length of a substrate, then typically there is nooverlapping of washcoat layers within the washcoat zone. Thus, thewashcoat zone typically comprises a discrete washcoat layer. Forexample, a washcoat zone may comprise, or consist of, two washcoatlayers (e.g. two complete washcoat layers), such as where one layer isdisposed on the other. The boundaries or edges of each washcoat layerare coterminous. In this context, the term “washcoat zone” is narrowerthan “washcoat region” because a “washcoat zone” does not comprise partof a washcoat layer.

When the “washcoat zone” refers to the arrangement of an impregnatedpart or portion of one or more washcoats along the length of asubstrate, the washcoat zone may be a part or portion of a singlewashcoat layer. Alternatively, the washcoat zone may be a part orportion of a plurality of washcoat layers, such as two washcoat layers.One of the boundaries or edges of the washcoat zone will be at an end ofthe substrate (e.g. an inlet end or an outlet end of the substrate).When the washcoat zone is a part or portion of a plurality of washcoatlayers, then the washcoat layers will have a common boundary or edge(i.e. along the length of the substrate).

The term “washcoat layer” used herein refers to a thickness of washcoatspread over a surface (e.g. a surface of the substrate or anotherwashcoat), which has a substantially uniform composition (i.e. there isno substantial difference in the composition of the washcoat whencomparing one part of the washcoat region with another part of thatwashcoat region). Substantially uniform composition in this contexttypically refers to a washcoat layer where the difference in compositionwhen comparing one part of the layer with another part of the layer is5% or less, usually 2.5% or less, and most commonly 1% or less. Thus, awashcoat layer is a discrete region or area of washcoat on thesubstrate.

Any reference to a washcoat region, zone or layer “disposed at an inletend of the substrate” used herein refers to a washcoat region, zone orlayer that is located on a substrate at a position that is nearer to aninlet end of the substrate than it is to an outlet end of the substrate.Thus, the midpoint of the washcoat region, zone or layer (i.e. at halfits length) is nearer to the inlet end of the substrate than themidpoint is to the outlet end of the substrate. Any reference to“disposed at an outlet end of the substrate” used herein refers to awashcoat region, zone or layer that is located on a substrate at aposition is nearer to an outlet end of the substrate than it is to aninlet end of the substrate. The midpoint of the washcoat region, zone orlayer (i.e. at half its length) is nearer to the outlet end of thesubstrate than the midpoint is to the inlet end of the substrate.

The expression “combination of platinum and palladium” as used hereinsimply refers to a washcoat region, layer or zone that contains bothplatinum and palladium. The “combination” includes, but is not limitedto, alloys or mixtures of platinum and palladium.

The term “mixed oxide” used herein generally refers to a mixture ofoxides in a single phase, as known in the art. The term “compositeoxide” used herein generally refers to a composition of oxides havingmore than one phase, as known in the art.

Any reference herein to an amount in units of g ft⁻³ (grams per cubicfoot) or g in⁻³ (grams per cubic inch) etc. refer to the mean weight ofa component per volume of the substrate.

The expression “consisting essentially” used herein limits the scope ofa feature to include the specified materials or steps, and any othermaterials or steps that do not materially affect the basiccharacteristics of that feature, such as for example minor impurities.The expression “consisting essentially of” embraces the expression“consisting of”.

Any reference to the length of a zone, layer or region used hereinrefers to its mean length. It is well known in the art that there may besome variation in the precise length of the zone, layer or regiondepending on the method used for its manufacture. Normally, the lengthdoes not deviate by more than 5%, preferably not more than 1%, from themean value of the length. The length of a zone, layer or region ismeasured parallel to the longitudinal axis (i.e. axial length) of theoxidation catalyst.

Reference is made herein to the lengths L₁, L₂ and L₃. Each of theselengths is measured from the end of the substrate that was coated withthe relevant washcoat (in the case of L₁ or L₂) or impregnated with aplatinum group metal (in the case of L₃). Any reference to a first endof the substrate relates to a different end of the substrate to thesecond end of the substrate. Normally, the first end of the substrate isthe opposite end of the substrate to the second end.

The expression “adsorber” or “adsorbent” as used herein is synonymouswith “absorber” or “absorbent”.

For the avoidance of doubt, the terms “length of the substrate” and“axial length of the substrate” are synonymous.

EXAMPLES

The invention will now be illustrated by the following non-limitingexamples.

Preparation of Catalysts

A zoned diesel oxidation catalyst (Zoned DOC) was prepared in accordancewith the second method aspect of the invention and as described in WO99/47260. The zoned (DOC) has a structure as shown in FIG. 4. The lengthof the first washcoat region (1) (e.g. L−L₂) was 25% of the length ofthe substrate (e.g. L). The loading of platinum was 26.7 g ft⁻³ and theloading of palladium was 26.7 g ft⁻³ in the first washcoat region. Thelength of the second washcoat region (2) (e.g. L₂+L₁−L) was 20% of thelength of the substrate. The loading of platinum was 34.7 g ft⁻³ and theloading of palladium was 26.7 g ft⁻³ in the second washcoat region. Thelength of the third washcoat region (3) (e.g. L−L₁) was 55% of thelength of the substrate. The loading of platinum was 8 g ft⁻³ in thethird washcoat region and no palladium was present. The overall loadingof PGM (e.g. platinum and palladium) of the catalyst was 30 g ft⁻³. Theratio of the total mass of platinum to the total mass of palladium inthe oxidation catalyst was 1.5:1.

A reference diesel oxidation catalyst (Reference DOC) was prepared usinga method as described in WO 99/47260 for comparative purposes. Thereference oxidation catalyst had two washcoat layers, which were eachcoated along the entire length of the substrate. A first layer that wascoated directly onto the substrate contained a platinum loading of 16 gft⁻³ and a palladium loading of 16 g ft⁻³. A second layer was coatedonto the first layer and contained a platinum loading of 8 g ft⁻³. Theoverall loading of PGM (e.g. platinum and palladium) of the catalyst was40 g ft⁻³. The ratio of the total mass of platinum to the total mass ofpalladium in the oxidation catalyst was 1.5:1.

NO Oxidation Activity

The NO oxidation activity of the Zoned DOC and the Reference DOC weremeasured and the results are shown in FIG. 5. The Zoned DOC (▴) showsgreater NO oxidation activity than the Reference DOC (♦) even though theZoned DOC has a lower total PGM loading of 30 g ft⁻³ than the ReferenceDOC, which has a total PGM loading of 40 g ft⁻³.

Hydrocarbon Oxidation Activity

The Zoned DOC and the Reference DOC were tested before and after ageingon a 7 L IT4 engine at 610° C. for 50 and 100 hours in a DOC+CSF filterconfiguration. The Zoned DOC and Reference DOC were compared in a quenchtest. Diesel fuel was injected over each catalyst to generate anexotherm, so that the exhaust gas temperature leaving the catalyst was610° C. Exotherm generation was stabilized at steady state temperatureand flow conditions (speed and load of the engine) and then the load ofthe engine was reduced so that the catalyst inlet temperature wasreduced. When the catalyst inlet temperature becomes too low to sustainexotherm generation the catalyst quenches. The results are shown inFIGS. 6 and 7. As can be seen in FIG. 6, the Reference DOC quenches at270° C. In contrast, the Zoned DOC does not quench under the sameconditions (see FIG. 7), which indicates that it has superior HCoxidation activity compared to the Reference DOC. The Zoned DOCmaintains its HC oxidation performance at lower temperatures than theReference DOC. This indicates that the Zoned DOC can be used toregenerate emissions control devices comprising a filtering substrate atlow operating temperatures. The HC slip from each catalyst was alsotested under the same catalyst inlet temperature and flow conditions.The results shown in FIG. 8 were obtained with an exhaust flow of about880 kg h⁻¹, a GHSV of 75 k h⁻¹, the catalyst inlet and outlettemperatures were 340 and 610° C. respectively. The results shown inFIG. 9 were obtained with an exhaust flow of about 1050 kg h⁻¹, a GHSVof 115 k h⁻¹, the catalyst inlet and outlet temperatures were 330 and610° C. respectively. The Zoned DOC shows lower HC slip than theReference DOC.

Sulphur Tolerance

The Zoned DOC and the Reference DOC were each sulphated with low Sdiesel (1.5 g L⁻¹ of sulphur) and the NO oxidation performance wasmeasured at 275° C. It can be seen from FIG. 10 that the Zoned DOC losesless NO oxidation performance after S exposure than the Reference DOC.The Zoned DOC also recovers fully after regeneration, while theReference DOC recovers partially after regeneration at the sametemperature. The HC slip during regeneration for each catalyst did notchange after sulphation.

For the avoidance of any doubt, the entire content of any and alldocuments cited herein is incorporated by reference into the presentapplication.

The invention claimed is:
 1. An oxidation catalyst for treating anexhaust gas from a diesel engine, which oxidation catalyst comprises: asubstrate; a first washcoat region disposed on the substrate, whereinthe first washcoat region comprises a first platinum group metal (PGM)and a first support material; a second washcoat region adjacent to thefirst washcoat region, wherein the second washcoat region comprises asecond platinum group metal (PGM) and a second support material; a thirdwashcoat region disposed on the substrate, wherein the third washcoatregion comprises a third platinum group metal (PGM) and a third supportmaterial; and wherein the second washcoat region is disposed on thethird washcoat region, and further wherein: (a) the total loading of thesecond PGM in the second washcoat region is greater than the totalloading of the first PGM in the first washcoat region; and/or (b) thetotal loading of the second PGM in the second washcoat region is greaterthan the total loading of the third PGM in the third washcoat region;and/or (c) the total loading of the first PGM in the first washcoatregion is greater than the total loading of the third PGM in the thirdwashcoat region.
 2. The oxidation catalyst according to claim 1, whereinthe first washcoat region comprises a first washcoat layer or a firstwashcoat zone, the second washcoat region comprises a second washcoatlayer, and the third washcoat region comprises a third washcoat layer.3. The oxidation catalyst according to claim 1, wherein the firstwashcoat region consists of a first washcoat layer.
 4. The oxidationcatalyst according to claim 2, wherein the first washcoat zone comprisesa first washcoat layer and a fourth washcoat layer, wherein the fourthwashcoat layer is disposed on the first washcoat layer.
 5. The oxidationcatalyst according to claim 1, wherein the total loading of the secondPGM in the second washcoat region is greater than the total loading ofthe third PGM in the third washcoat region.
 6. The oxidation catalystaccording to claim 1, wherein the total loading of the first PGM in thefirst washcoat region is greater than the total loading of the third PGMin the third washcoat region.
 7. The oxidation catalyst according toclaim 1, wherein the total loading of the second PGM in the secondwashcoat region is greater than the total loading of the first PGM inthe first washcoat region.
 8. The oxidation catalyst according to claim1, wherein the first PGM is selected from the group consisting ofplatinum and a combination of platinum and palladium.
 9. The oxidationcatalyst according to claim 1, wherein the second PGM is selected fromthe group consisting of platinum and a combination of platinum andpalladium.
 10. The oxidation catalyst according to claim 1, wherein thethird PGM is selected from the group consisting of platinum and acombination of platinum and palladium.
 11. The oxidation catalystaccording to claim 1, wherein the first support material comprises arefractory metal oxide selected from the group consisting of alumina,silica, titania, zirconia, ceria and mixed or composite oxides of two ormore thereof.
 12. The oxidation catalyst according to claim 1, whereinthe second support material comprises a refractory metal oxide selectedfrom the group consisting of alumina, silica, titania, zirconia, ceriaand mixed or composite oxides of two or more thereof.
 13. The oxidationcatalyst according to claim 1, wherein the third support materialcomprises a refractory metal oxide selected from the group consisting ofalumina, silica, titania, zirconia, ceria and mixed or composite oxidesof two or more thereof.
 14. An exhaust system comprising an oxidationcatalyst according to claim 1, and an emissions control device.
 15. Avehicle comprising a diesel engine and the exhaust system according toclaim
 14. 16. The oxidation catalyst according to claim 1, wherein thesecond washcoat region has a ratio by total weight of manganese (Mn) toplatinum in a range of from of 5:1 to 0.2:1.
 17. The oxidation catalystaccording to claim 1, wherein the second washcoat region comprisespalladium.
 18. The oxidation catalyst according to claim 1, wherein thesecond washcoat region has a ratio of platinum to palladium by totalweight of from 1:0 to 2:1.
 19. The oxidation catalyst according to claim1, wherein at least one of the first support material, the secondsupport material, or the third support material comprises alumina dopedwith silica or silica-alumina.
 20. The oxidation catalyst according toclaim 1, wherein the first washcoat region further comprises at leastone of an alkali metal, alkaline earth metal and a rare earth metalselected from the group consisting of lanthanum, yttrium and acombination thereof.
 21. The oxidation catalyst according to claim 1,wherein the first PGM is disposed or supported on the first supportmaterial.
 22. The oxidation catalyst according to claim 1, wherein thefirst support material comprises a refractory metal oxide comprising analkaline earth metal aluminate.
 23. The oxidation catalyst according toclaim 1, wherein the first support material comprises a refractory metaloxide selected from the group consisting of alumina, silica, titania,zirconia, ceria and a mixed or composite oxide thereof, optionally dopedwith a dopant.
 24. The oxidation catalyst according to claim 1, whereinthe first PGM is a combination of platinum and palladium, and wherein atleast one of the platinum and the palladium is disposed or supported onthe first support material.
 25. The oxidation catalyst according toclaim 1, wherein the first PGM is a combination of platinum andpalladium, a combination of palladium and rhodium or a combination ofplatinum, palladium and rhodium, and wherein the first washcoat regionfurther comprises a palladium support material.
 26. The oxidationcatalyst according to claim 25, wherein at least one of the platinum andthe rhodium is disposed or supported on the first support material andthe palladium is disposed or supported on the palladium supportmaterial.
 27. The oxidation catalyst according to claim 26, wherein thepalladium support material comprises a refractory metal oxide comprisingceria or a mixed or composite oxide of ceria.
 28. The oxidation catalystaccording to claim 1, wherein the substrate is a through-flow substrate.29. The oxidation catalyst according to claim 4, wherein the totalloading of the first PGM is greater than the total loading of each ofthe second PGM and the third PGM.